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                <h2 id="1-概述"><a href="#1-概述" class="headerlink" title="1. 概述"></a>1. 概述</h2><p><strong>在网络中传递的数据总是具有相同的类型：字节</strong>。 这些字节流动的细节取决于网络传输，它是一个帮我们抽象底层数据传输机制的概念，我们不需要关心字节流动的细节，只需要确保字节被可靠的接收和发送。</p>
<p>当我们使用 Java 网络编程时，可能会接触到多种不同的网络 IO 模型，如 NIO，BIO (OIO: Old IO)，AIO 等，我们可能因为使用这些不同的 API 而遇到问题。 <strong>Netty 则为这些不同的 IO 模型实现了一个通用的 API</strong>，<u>我们使用这个通用的 API 比直接使用 JDK 提供的 API 要简单的多，且避免了由于使用不同 API 而带来的问题，大大提高了代码的可读性</u>。 在传输这一部分，我们将主要学习这个通用的 API，以及它与 JDK 之间的对比。</p>
<h2 id="2-传输-API-—-Channel"><a href="#2-传输-API-—-Channel" class="headerlink" title="2. 传输 API — Channel"></a>2. 传输 API — Channel</h2><h3 id="①-概述"><a href="#①-概述" class="headerlink" title="① 概述"></a>① 概述</h3><p>传输 API 的核心是 <code>Channel</code> 接口 (<code>io.netty.Channel</code>，而非 java nio 的 <code>Channel</code>) ，它被用于所有的 IO 操作。</p>
<p><code>Channel</code> 结构层次：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115933118.png" alt="image-20220327115933118" style="zoom:80%;" />

<p><strong>每个<code>Channel</code>都会被分配一个<code>ChannelPipeline</code>和<code>ChannelConfig</code>：</strong></p>
<ul>
<li><p><code>ChannelConfig</code>包含了该<code>Channel</code>的所有配置，并允许在运行期间更新它们。</p>
</li>
<li><p><code>ChannelPipeline </code>存储了所有用于处理出站和入站数据的 <code>ChannelHandler</code>， 我们可以在运行时根据自己的需求添加或删除<code>ChannelPipeline</code>中的<code>ChannelHandler</code>。</p>
</li>
</ul>
<p>此外，<code>Channel</code> 还有以下方法值得留意：</p>
<table>
<thead>
<tr>
<th align="center">方法名</th>
<th align="center">描述</th>
</tr>
</thead>
<tbody><tr>
<td align="center"><code>eventLoop</code></td>
<td align="center">返回当前Channel注册到的EventLoop</td>
</tr>
<tr>
<td align="center"><code>pipeline</code></td>
<td align="center">返回分配给Channel的ChannelPipeline</td>
</tr>
<tr>
<td align="center"><code>isActive</code></td>
<td align="center">判断当前Channel是活动的，如果是则返回true。 此处活动的意义依赖于底层的传输，如果底层传输是TCP Socket，那么客户端与服务端保持连接便是活动的；如果底层传输是UDP Datagram，那么Datagram传输被打开就是活动的。</td>
</tr>
<tr>
<td align="center"><code>localAddress</code></td>
<td align="center">返回本地SocketAddress</td>
</tr>
<tr>
<td align="center"><code>remoteAddress</code></td>
<td align="center">返回远程的SocketAddress</td>
</tr>
<tr>
<td align="center"><code>write</code></td>
<td align="center">将数据写入远程主机，数据将会通过ChannelPipeline传输</td>
</tr>
<tr>
<td align="center"><code>flush</code></td>
<td align="center">将之前写入的数据刷新到底层传输</td>
</tr>
<tr>
<td align="center"><code>writeFlush</code></td>
<td align="center">等同于调用 write 写入数据后再调用 flush 刷新数据</td>
</tr>
</tbody></table>
<p>举个例子：写数据到远程已连接客户端可以调用 <code>Channel.write()</code> 方法，如下代码：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token class-name">Channel</span> channel <span class="token operator">=</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">;</span> <span class="token comment">// 获取 channel 的引用</span>
<span class="token comment">// 创建 ByteBuf 保存写的数据</span>
<span class="token class-name">ByteBuf</span> buf <span class="token operator">=</span> <span class="token class-name">Unpooled</span><span class="token punctuation">.</span><span class="token function">copiedBuffer</span><span class="token punctuation">(</span><span class="token string">"your data"</span><span class="token punctuation">,</span> <span class="token class-name">CharsetUtil</span><span class="token punctuation">.</span>UTF_8<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 写数据，并刷新</span>
<span class="token class-name">ChannelFuture</span> cf <span class="token operator">=</span> channel<span class="token punctuation">.</span><span class="token function">writeAndFlush</span><span class="token punctuation">(</span>buf<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 添加 ChannelFutureListener 监听，即可在写操作完成后收到通知，</span>
cf<span class="token punctuation">.</span><span class="token function">addListener</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">ChannelFutureListener</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">operationComplete</span><span class="token punctuation">(</span><span class="token class-name">ChannelFuture</span> future<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>future<span class="token punctuation">.</span><span class="token function">isSuccess</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> <span class="token comment">// 写操作顺利完成</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"Write successful"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span> <span class="token keyword">else</span> <span class="token punctuation">&#123;</span> <span class="token comment">// 写操作完成时出现错误</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>err<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"Write error"</span><span class="token punctuation">)</span><span class="token punctuation">;</span> 
            future<span class="token punctuation">.</span><span class="token function">cause</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p><strong><code>Channel</code> 是线程安全(thread-safe)的，它可以被多个不同的线程安全的操作</strong>，在多线程环境下，所有的方法都是安全的。正因为 <code>Channel </code>是安全的，我们存储对<code>Channel</code>的引用，并在学习的时候使用它写入数据到远程已连接的客户端，使用多线程也是如此。下面的代码是一个简单的<strong>多线程</strong>例子：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">final</span> <span class="token class-name">Channel</span> channel <span class="token operator">=</span> <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">;</span> <span class="token comment">// 获取channel的引用</span>
<span class="token comment">// 创建一个 ByteBuf 保存写的数据</span>
<span class="token keyword">final</span> <span class="token class-name">ByteBuf</span> buf <span class="token operator">=</span> <span class="token class-name">Unpooled</span><span class="token punctuation">.</span><span class="token function">copiedBuffer</span><span class="token punctuation">(</span><span class="token string">"your data"</span><span class="token punctuation">,</span> <span class="token class-name">CharsetUtil</span><span class="token punctuation">.</span>UTF_8<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">retain</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 创建 Runnable 用于写数据到 channel</span>
<span class="token class-name">Runnable</span> writer <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Runnable</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span> 
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        channel<span class="token punctuation">.</span><span class="token function">writeAndFlush</span><span class="token punctuation">(</span>buf<span class="token punctuation">.</span><span class="token function">duplicate</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span class="token punctuation">;</span>
<span class="token comment">// 获取 Executor 的引用使用线程来执行任务</span>
<span class="token class-name">Executor</span> executor <span class="token operator">=</span> <span class="token class-name">Executors</span><span class="token punctuation">.</span><span class="token function">newCachedThreadPool</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// 写进一个线程</span>
executor<span class="token punctuation">.</span><span class="token function">execute</span><span class="token punctuation">(</span>writer<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">//写进另外一个线程</span>
executor<span class="token punctuation">.</span><span class="token function">execute</span><span class="token punctuation">(</span>writer<span class="token punctuation">)</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h3 id="②-直观感受-Netty-统一的传输-API"><a href="#②-直观感受-Netty-统一的传输-API" class="headerlink" title="② 直观感受 Netty 统一的传输 API"></a>② 直观感受 Netty 统一的传输 API</h3><p><strong>Netty 使用相同的 API 来实现每个传输，它并不关心你使用什么来实现</strong>。Netty 通过操作接口 <code>Channel </code>、<code>ChannelPipeline </code>和 <code>ChannelHandler</code>来实现。</p>
<p>下面我们通过直观的例子来感受下，Netty 只需要简单的修改就可以将 BIO 改为 NIO。</p>
<h4 id="Ⅰ-Netty-BIO-版本"><a href="#Ⅰ-Netty-BIO-版本" class="headerlink" title="Ⅰ Netty BIO 版本"></a>Ⅰ Netty BIO 版本</h4><p>下面代码是使用 Netty 作为网络框架编写的一个阻塞 IO 例子：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">NettyOioServer</span> <span class="token punctuation">&#123;</span>

    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">server</span><span class="token punctuation">(</span><span class="token keyword">int</span> port<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">Exception</span> <span class="token punctuation">&#123;</span>
        <span class="token keyword">final</span> <span class="token class-name">ByteBuf</span> buf <span class="token operator">=</span> <span class="token class-name">Unpooled</span><span class="token punctuation">.</span><span class="token function">unreleasableBuffer</span><span class="token punctuation">(</span>
                <span class="token class-name">Unpooled</span><span class="token punctuation">.</span><span class="token function">copiedBuffer</span><span class="token punctuation">(</span><span class="token string">"Hi!\r\n"</span><span class="token punctuation">,</span> <span class="token class-name">Charset</span><span class="token punctuation">.</span><span class="token function">forName</span><span class="token punctuation">(</span><span class="token string">"UTF-8"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">EventLoopGroup</span> group <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">OioEventLoopGroup</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">try</span> <span class="token punctuation">&#123;</span>
            <span class="token comment">// 创建一个引导类 ServerBootstrap</span>
            <span class="token class-name">ServerBootstrap</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ServerBootstrap</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

            b<span class="token punctuation">.</span><span class="token function">group</span><span class="token punctuation">(</span>group<span class="token punctuation">)</span>
             <span class="token comment">// 使用 OioEventLoopGroup 允许阻塞模式（Old-IO 即 BIO）</span>
             <span class="token punctuation">.</span><span class="token function">channel</span><span class="token punctuation">(</span><span class="token class-name">OioServerSocketChannel</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span> 
             <span class="token punctuation">.</span><span class="token function">localAddress</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">InetSocketAddress</span><span class="token punctuation">(</span>port<span class="token punctuation">)</span><span class="token punctuation">)</span>
             <span class="token comment">// 指定 ChannelInitializer 将给每个接受的连接调用</span>
             <span class="token punctuation">.</span><span class="token function">childHandler</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">ChannelInitializer</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">SocketChannel</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
                 <span class="token annotation punctuation">@Override</span>
                 <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">initChannel</span><span class="token punctuation">(</span><span class="token class-name">SocketChannel</span> ch<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">Exception</span> <span class="token punctuation">&#123;</span>
                     <span class="token comment">// 添加 ChannelHandler 拦截事件，并允许他们作出反应</span>
                     ch<span class="token punctuation">.</span><span class="token function">pipeline</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">addLast</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">ChannelInboundHandlerAdapter</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
                         <span class="token annotation punctuation">@Override</span>
                         <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">channelActive</span><span class="token punctuation">(</span><span class="token class-name">ChannelHandlerContext</span> ctx<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">Exception</span><span class="token punctuation">&#123;</span>
                             <span class="token comment">// 写信息到客户端，并添加监听：一旦消息写入就关闭连接</span>
                             ctx<span class="token punctuation">.</span><span class="token function">writeAndFlush</span><span class="token punctuation">(</span>buf<span class="token punctuation">.</span><span class="token function">duplicate</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">.</span>
                                 <span class="token function">addListener</span><span class="token punctuation">(</span><span class="token class-name">ChannelFutureListener</span><span class="token punctuation">.</span>CLOSE<span class="token punctuation">)</span><span class="token punctuation">;</span>
                         <span class="token punctuation">&#125;</span>
                     <span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                 <span class="token punctuation">&#125;</span>
             <span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token comment">// 绑定服务器来接受连接</span>
            <span class="token class-name">ChannelFuture</span> f <span class="token operator">=</span> b<span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">sync</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            f<span class="token punctuation">.</span><span class="token function">channel</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">closeFuture</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">sync</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span> <span class="token keyword">finally</span> <span class="token punctuation">&#123;</span>
            <span class="token comment">// 释放所有资源</span>
            group<span class="token punctuation">.</span><span class="token function">shutdownGracefully</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">sync</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>下面代码是使用 Netty NIO 实现 ：</p>
<h4 id="Ⅱ-Netty-NIO-版本"><a href="#Ⅱ-Netty-NIO-版本" class="headerlink" title="Ⅱ Netty NIO 版本"></a>Ⅱ Netty NIO 版本</h4><p>下面是 Netty NIO 的代码， 只是将 <code>OioEventLoopGroup</code> 变成了 <code>NioEventLoopGroup</code>，将 <code>OioServerSocketChannel</code> 变成了 <code>NioServerSocketChannel</code>，就从 BIO (OIO) 传输 切换到了 NIO。</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">NettyNioServer</span> <span class="token punctuation">&#123;</span>

    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">server</span><span class="token punctuation">(</span><span class="token keyword">int</span> port<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">Exception</span> <span class="token punctuation">&#123;</span>
        <span class="token keyword">final</span> <span class="token class-name">ByteBuf</span> buf <span class="token operator">=</span> <span class="token class-name">Unpooled</span><span class="token punctuation">.</span><span class="token function">unreleasableBuffer</span><span class="token punctuation">(</span>
                <span class="token class-name">Unpooled</span><span class="token punctuation">.</span><span class="token function">copiedBuffer</span><span class="token punctuation">(</span><span class="token string">"Hi!\r\n"</span><span class="token punctuation">,</span> <span class="token class-name">Charset</span><span class="token punctuation">.</span><span class="token function">forName</span><span class="token punctuation">(</span><span class="token string">"UTF-8"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">NioEventLoopGroup</span> group <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">NioEventLoopGroup</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">try</span> <span class="token punctuation">&#123;</span>
            <span class="token comment">// 创建一个引导类 ServerBootstrap</span>
            <span class="token class-name">ServerBootstrap</span> b <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ServerBootstrap</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            b<span class="token punctuation">.</span><span class="token function">group</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">NioEventLoopGroup</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token keyword">new</span> <span class="token class-name">NioEventLoopGroup</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
             <span class="token comment">// 使用 NioEventLoopGroup 允许NIO模式</span>
             <span class="token punctuation">.</span><span class="token function">channel</span><span class="token punctuation">(</span><span class="token class-name">NioServerSocketChannel</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span>
             <span class="token punctuation">.</span><span class="token function">localAddress</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">InetSocketAddress</span><span class="token punctuation">(</span>port<span class="token punctuation">)</span><span class="token punctuation">)</span>
             <span class="token comment">// 指定 ChannelInitializer 将给每个接受的连接调用</span>
             <span class="token punctuation">.</span><span class="token function">childHandler</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">ChannelInitializer</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">SocketChannel</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
                 <span class="token annotation punctuation">@Override</span>
                 <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">initChannel</span><span class="token punctuation">(</span><span class="token class-name">SocketChannel</span> ch<span class="token punctuation">)</span> 
                     <span class="token keyword">throws</span> <span class="token class-name">Exception</span> <span class="token punctuation">&#123;</span>
                     <span class="token comment">// 添加 ChannelHandler 拦截事件，并允许他们作出反应</span>
                     ch<span class="token punctuation">.</span><span class="token function">pipeline</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">addLast</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">ChannelInboundHandlerAdapter</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
                         <span class="token annotation punctuation">@Override</span>
                         <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">channelActive</span><span class="token punctuation">(</span><span class="token class-name">ChannelHandlerContext</span> ctx<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">Exception</span><span class="token punctuation">&#123;</span>
                             <span class="token comment">// 写信息到客户端，并添加监听：一旦消息写入就关闭连接</span>
                             ctx<span class="token punctuation">.</span><span class="token function">writeAndFlush</span><span class="token punctuation">(</span>buf<span class="token punctuation">.</span><span class="token function">duplicate</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
                                <span class="token punctuation">.</span><span class="token function">addListener</span><span class="token punctuation">(</span><span class="token class-name">ChannelFutureListener</span><span class="token punctuation">.</span>CLOSE<span class="token punctuation">)</span><span class="token punctuation">;</span>
                         <span class="token punctuation">&#125;</span>
                     <span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                 <span class="token punctuation">&#125;</span>
             <span class="token punctuation">&#125;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token comment">// 绑定服务器来接受连接</span>
            <span class="token class-name">ChannelFuture</span> f <span class="token operator">=</span> b<span class="token punctuation">.</span><span class="token function">bind</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">sync</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> 
            f<span class="token punctuation">.</span><span class="token function">channel</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">closeFuture</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">sync</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span> <span class="token keyword">finally</span> <span class="token punctuation">&#123;</span>
            <span class="token comment">// 释放所有资源</span>
            group<span class="token punctuation">.</span><span class="token function">shutdownGracefully</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">sync</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h2 id="3-Netty-自带的传输方式-协议"><a href="#3-Netty-自带的传输方式-协议" class="headerlink" title="3. Netty 自带的传输方式/协议"></a>3. Netty 自带的传输方式/协议</h2><p>Netty 自带了一些传输协议的实现，虽然没有支持所有的传输协议，但是其自带的已足够我们来使用。Netty 应用程序的传输协议依赖于底层协议。</p>
<p>Netty内置<code>Channel</code>接口层次：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115941706.png" alt="image-20220327115941706" style="zoom:80%;" />

<p>Netty 中的传输方式有如下几种：</p>
<table>
<thead>
<tr>
<th align="center">名称</th>
<th align="center">包</th>
<th align="left">描述</th>
</tr>
</thead>
<tbody><tr>
<td align="center">NIO</td>
<td align="center">io.netty.channel.socket.nio</td>
<td align="left"><code>NioChannel</code>基于java.nio.channels，其io模型为IO多路复用</td>
</tr>
<tr>
<td align="center">Epoll</td>
<td align="center">io.netty.channel.epoll</td>
<td align="left"><code>EpollChannel</code>基于操作系统的epoll函数，其io模型为IO多路复用，不过Epoll模型只支持在Linux上的多种特性，比NIO性能更好</td>
</tr>
<tr>
<td align="center">KQueue</td>
<td align="center">io.netty.channel.kqueue</td>
<td align="left"><code>KQueue </code>与 Epoll 相似，它主要被用于 FreeBSD 系统上，如Mac等</td>
</tr>
<tr>
<td align="center">OIO(Old Io)</td>
<td align="center">io.netty.channel.socket.oio</td>
<td align="left"><code>OioChannel</code>基于 java.net 包，其 io 模型是阻塞的，且此传输被 Netty 标记为 deprecated，故不推荐使用，最好使用 NIO / EPOLL / KQUEUE 等传输</td>
</tr>
<tr>
<td align="center">Local</td>
<td align="center">io.netty.channel.local</td>
<td align="left"><code>LocalChannel </code>可以在 JVM 虚拟机内部进行本地通信</td>
</tr>
<tr>
<td align="center">Embedded</td>
<td align="center">io.netty.channel.embedded</td>
<td align="left"><code>EmbeddedChannel</code>允许在没有真正的网络传输中使用ChannelHandler，可以非常有用的测试 ChannelHandler</td>
</tr>
</tbody></table>
<h2 id="4-零拷贝"><a href="#4-零拷贝" class="headerlink" title="4. 零拷贝"></a>4. 零拷贝</h2><h3 id="①-什么是零拷贝"><a href="#①-什么是零拷贝" class="headerlink" title="① 什么是零拷贝"></a>① 什么是零拷贝</h3><p><strong>零拷贝(Zero-Copy)<strong>是一种目前只有在使用<code>NIO</code>和<code>Epoll</code>传输时才可使用的特性。 在之前的 IO 模型中，所有的 IO 的数据都是从内核复制到用户应用进程，再由用户应用进程处理。 而</strong>零拷贝则可以快速地将数据从源文件移动到目标文件，无需经过用户空间</strong>。</p>
<p>在学习零拷贝技术之前先回顾一下普通的 IO 拷贝过程吧， 这里举个栗子： 我要使用一个程序将一个目录下的文件复制到另一个目录下， 在普通的 IO 中，其过程如下：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115948341.png" alt="image-20220327115948341" style="zoom:80%;" />

<ul>
<li>应用程序启动后，向内核发出 <code>read</code> 调用（用户态切换到内核态）</li>
<li>操作系统收到调用请求后， 会检查文件是否已经缓存过了：<ul>
<li>如果缓存过了，就将数据从缓冲区（直接内存）拷贝到用户应用进程（内核态切换到用户态）</li>
<li>如果是第一次访问这个文件，则系统先将数据先拷贝到缓冲区（直接内存），然后CPU将数据从缓冲区拷贝到应用进程内（内核态切换到用户态）</li>
</ul>
</li>
<li>应用进程收到内核的数据后发起 <code>write</code> 调用，将数据拷贝到目标文件相关的堆栈内存（用户态切换到内核态）， 最后再从缓存拷贝到目标文件。</li>
</ul>
<p>根据上面普通拷贝的过程我们知道了其缺点主要有：</p>
<ol>
<li>用户态与内核态之间的上下文切换次数较多（用户态发送系统调用与内核态将数据拷贝到用户空间）。</li>
<li>拷贝次数较多，每次 IO 都需要 DMA 和 CPU 拷贝。</li>
</ol>
<p><strong>而零拷贝正是针对普通拷贝的缺点做了很大改进，使得其拷贝速度在处理大数据的时候很是出色</strong>。</p>
<h3 id="②-如何实现零拷贝"><a href="#②-如何实现零拷贝" class="headerlink" title="② 如何实现零拷贝"></a>② 如何实现零拷贝</h3><p>零拷贝主要有两种实现技术：</p>
<ol>
<li>内存映射</li>
<li>文件传输</li>
</ol>
<h4 id="Ⅰ-内存映射-Memory-Mapped"><a href="#Ⅰ-内存映射-Memory-Mapped" class="headerlink" title="Ⅰ 内存映射 Memory Mapped"></a>Ⅰ 内存映射 Memory Mapped</h4><p>内存映射对应 JAVA NIO 的 API 为 <code>FileChannel.map</code>。</p>
<p>当用户程序发起 <code>mmp</code> 系统调用后，操作系统会将文件的数据直接映射到内核缓冲区中， 且<strong>缓冲区会与用户空间共享这一块内存</strong>，这样就无需将数据从内核拷贝到用户空间了，用户程序接着发起 <code>write</code> 调用，操作系统直接将内核缓冲区的数据拷贝到目标文件的缓冲区，最后再将数据从缓冲区拷贝到目标文件。</p>
<p>其过程如下：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327115955954.png" alt="image-20220327115955954" style="zoom:80%;" />

<p><strong>内存映射由原来的四次拷贝减少到了三次，且拷贝过程都在内核空间，这在很大程度上提高了 IO 效率</strong>。</p>
<p>但是 <code>mmp</code> 也有缺点： 当我们使用 <code>mmp</code> 调用映射一个文件到内存后，如果另一个进程同时对这个文件阶段或是做出写的操作， 那么系统如果此时正在将数据 <code>write</code> 到目标文件，用户程序可能会因为访问非法地址而产生一个错误的信号从而终止。</p>
<p>试想一种情况：我们的服务器接收一个客户端的下载请求，客户端请求的是一个超大的文件，服务端开启一个线程 使用 <code>mmp</code> 和 <code>write</code> 将文件拷贝到 Socket 进行响应，如果此时又有一个客户端请求对这个文件做出修改， 由于这个文件先前已经被第一个线程 <code>mmp</code> 了，可能第一个线程会因此出现异常，客户端也会请求失败。</p>
<p>解决这个问题的最简单的一种方法就对这个文件加读写锁，当一个线程对这个文件进行读或写时，其他线程不能操作此文件， 不过这样处理并发的能力可能就大打折扣了。</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">import</span> <span class="token namespace">io<span class="token punctuation">.</span>netty<span class="token punctuation">.</span>channel<span class="token punctuation">.</span></span><span class="token class-name">EventLoop</span><span class="token punctuation">;</span>

<span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>io<span class="token punctuation">.</span></span><span class="token class-name">IOException</span><span class="token punctuation">;</span>
<span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>nio<span class="token punctuation">.</span></span><span class="token class-name">MappedByteBuffer</span><span class="token punctuation">;</span>
<span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>nio<span class="token punctuation">.</span>channels<span class="token punctuation">.</span></span><span class="token class-name">FileChannel</span><span class="token punctuation">;</span>
<span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>nio<span class="token punctuation">.</span>file<span class="token punctuation">.</span></span><span class="token class-name">Path</span><span class="token punctuation">;</span>
<span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>nio<span class="token punctuation">.</span>file<span class="token punctuation">.</span></span><span class="token class-name">StandardOpenOption</span><span class="token punctuation">;</span>

<span class="token comment">/**
 * @description 零拷贝测试
 */</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">ZeroCopyTest</span><span class="token punctuation">&#123;</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">Exception</span><span class="token punctuation">&#123;</span>
		<span class="token function">mmp</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>

    <span class="token comment">/**
     * mmp 测试
     */</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">mmp</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">IOException</span><span class="token punctuation">&#123;</span>
        <span class="token comment">// 读文件通道</span>
        <span class="token class-name">FileChannel</span> fileChannelIn <span class="token operator">=</span> <span class="token class-name">FileChannel</span><span class="token punctuation">.</span><span class="token keyword">open</span><span class="token punctuation">(</span><span class="token class-name">Path</span><span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span><span class="token string">"/home/images/SC01C25.jpg"</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token class-name">StandardOpenOption</span><span class="token punctuation">.</span>READ<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment">// 写文件通道</span>
        <span class="token class-name">FileChannel</span> fileChannelTo <span class="token operator">=</span> <span class="token class-name">FileChannel</span><span class="token punctuation">.</span><span class="token keyword">open</span><span class="token punctuation">(</span><span class="token class-name">Path</span><span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span><span class="token string">"/home/images/a.jpg"</span><span class="token punctuation">)</span><span class="token punctuation">,</span><span class="token class-name">StandardOpenOption</span><span class="token punctuation">.</span>WRITE<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment">// 内存映射</span>
        <span class="token class-name">MappedByteBuffer</span> mappedByteBuffer <span class="token operator">=</span> fileChannelIn<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token class-name">FileChannel<span class="token punctuation">.</span>MapMode</span><span class="token punctuation">.</span>READ_ONLY<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> fileChannelIn<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        fileChannelTo<span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>mappedByteBuffer<span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token comment">// 如果是处理网络请求</span>
	   <span class="token comment">// SocketChannel socketChannel = SocketChannel.open();</span>
	   <span class="token comment">// SocketChannel.write(mappedByteBuffer);</span>

        fileChannelIn<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        fileChannelTo<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h4 id="Ⅱ-文件传输-Send-File"><a href="#Ⅱ-文件传输-Send-File" class="headerlink" title="Ⅱ 文件传输 Send File"></a>Ⅱ 文件传输 Send File</h4><p>文件传输对应 JAVA NIO 的 API 为 <code>FileChannel.transferFrom/transferTo</code></p>
<p>在了解文件传输之前，先来看一下它的函数原型（linux 系统的同学可以使用 man sendfile 查看）：</p>
<pre class="line-numbers language-text" data-language="text"><code class="language-text">#include&lt;sys/sendfile.h>

ssize_t 

sendfile(int out_fd,
        int in_fd,
        off_t *offset,
        size_t count);<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p><strong><code>sendfile</code> 在代表输入文件的文件描述符 <code>in_fd</code> 和 输出文件的文件描述符 <code>out_fd</code> 之间传输文件内容</strong>， 这个传输过程完全是在内核之中进行的，程序只需要把输入文件的描述符和输出文件的描述符传递给 <code>sendfile</code> 调用，系统自然会完成拷贝。 当然，<code>sendfile</code> 和 <code>mmp</code> 一样都有相同的缺点，在传输过程中， 如果有其他进程截断了这个文件的话，用户程序仍然会被终止。</p>
<p><code>sendfile</code> 传输过程如下：</p>
<img src="https://cdn.jsdelivr.net/gh/kuangtf/PictureBed/img/image-20220327120002759.png" alt="image-20220327120002759" style="zoom:80%;" />

<p>它的拷贝次数与 mmp 一样，但是 <strong>sendfile 的拷贝过程只在内核空间进行</strong>，无需像 mmp 一样与用户进程共享内存。</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>io<span class="token punctuation">.</span></span><span class="token class-name">IOException</span><span class="token punctuation">;</span>
<span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>nio<span class="token punctuation">.</span></span><span class="token class-name">MappedByteBuffer</span><span class="token punctuation">;</span>
<span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>nio<span class="token punctuation">.</span>channels<span class="token punctuation">.</span></span><span class="token class-name">FileChannel</span><span class="token punctuation">;</span>
<span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>nio<span class="token punctuation">.</span>file<span class="token punctuation">.</span></span><span class="token class-name">Path</span><span class="token punctuation">;</span>
<span class="token keyword">import</span> <span class="token namespace">java<span class="token punctuation">.</span>nio<span class="token punctuation">.</span>file<span class="token punctuation">.</span></span><span class="token class-name">StandardOpenOption</span><span class="token punctuation">;</span>

<span class="token comment">/**
 * @description 零拷贝测试
 */</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">ZeroCopyTest</span><span class="token punctuation">&#123;</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">Exception</span><span class="token punctuation">&#123;</span>
		<span class="token function">sendFile</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>

    <span class="token comment">/**
     * sendfile测试
     */</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">sendFile</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">IOException</span>
    <span class="token punctuation">&#123;</span>
        <span class="token comment">// 读文件通道</span>
        <span class="token class-name">FileChannel</span> fileChannelIn <span class="token operator">=</span> <span class="token class-name">FileChannel</span><span class="token punctuation">.</span><span class="token keyword">open</span><span class="token punctuation">(</span><span class="token class-name">Path</span><span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span><span class="token string">"/home/images/图片/壁纸/SC01C25.jpg"</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token class-name">StandardOpenOption</span><span class="token punctuation">.</span>READ<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment">// 写文件通道</span>
        <span class="token class-name">FileChannel</span> fileChannelTo <span class="token operator">=</span> <span class="token class-name">FileChannel</span><span class="token punctuation">.</span><span class="token keyword">open</span><span class="token punctuation">(</span><span class="token class-name">Path</span><span class="token punctuation">.</span><span class="token function">of</span><span class="token punctuation">(</span><span class="token string">"/home/images/a.jpg"</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token class-name">StandardOpenOption</span><span class="token punctuation">.</span>WRITE<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment">// fileChannelIn ——> fileChannelTo（sendfile)</span>
        fileChannelIn<span class="token punctuation">.</span><span class="token function">transferTo</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span>fileChannelIn<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> fileChannelTo<span class="token punctuation">)</span><span class="token punctuation">;</span>

        fileChannelIn<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        fileChannelTo<span class="token punctuation">.</span><span class="token function">close</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h2 id="引用"><a href="#引用" class="headerlink" title="引用"></a>引用</h2><blockquote>
<p>《Netty 实战》</p>
</blockquote>

                
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